Thermionic tube amplifier



April 22, 1941.

R. W. GILBERT THERMIONIC TUBE AMPLIFIER Filed Aug, 10, 1937 EELAT/ ve EEsPo/VSE zl/Wg Patented Apr. 22, 1941 2389362 'rHERMIONIo TUBE AMPLIFIER Roswell W. Gilbert, East Orange, N. J., assignor to Weston Electrical Instrument Corporation,

Newark, N. J., a ccrporation of New Jersey Application August 10, 1937, Serial No. 158,423

(Cl. ZSO-41.5)

Claims.

This invention relates to -thermionic tube amplifiers, and more particularly to direct current amplifiers.

Thermionic tubes have been employed for the amplification of direct currents by passing the current to be amplified through a grid-cathode resistor, thus producing a potential drop between the grid and cathode which, by the usual action of an amplifier tube. results in a change in. the plate current. It has been the practice to apply a steady negative'bias potential to the grid to insure negligible grid current relative to the order of the input current, and to permit adjustment of the Operating point to an optimum portion of the tube characteristic. The usual plate voltages provided a substantial fiow of plate current at zero input current and the usual practice has been to shunt the measuring instrument portion of the plate circuit by an adjustable voltage source to buck out this normal or zero input current flow through the measuring instrument. Zero stability and the amplification factor in such systems are dependent upon the settings of the grid reference voltage and the plate compensation current, and these adjustments are often critical and variable.` i

Objects of the invention are to provide simpler and more stable methods of and circuits for employing thermionic tubes as direct current amplifiers. amplifiers in which the space path resistance of the control grid against the cathode forms the load resistance for the current to be amplified. A further object is to provide a thermionic direct :urrent amplifier in which the output or plate zurrent falls to zero upon the cessation of a di- ,'ect current input to the amplifier. Other obects relate to the provision of novel amplifier :ircuits in which a direct current source such as i photo-electric device is connected directly Lcross the electrodes of a thermionic amplifier `ube.

These and other objects and advantages of the nvention will be apparent from the following Deciflcation when taken with the accompanying" lrawing in which: j

Fig. 1 is a schematic circuit diagram for ex- |lanatory purposes;

Fig. 2 is a circuit diagram of an embodiment f the invention; and

Fig. 3 is a curve sheet showing the relative reponse characteristic of the direct current am- Ififll'. i

In the schematic diagram of Fig. 1, the refrence numeral I identifles a vacuum tube hav- Objects are to provide direct current ing a hot cathode K, control grid G and plate P The plate circuit includes a. source of current 2 and a Sensitive direct current measuring instrument 3, but the grid and cathode are not connected externally of the tube. The space current path between grid G and cathode K is represented by an equivalent' but fictitious circuit, as is indicated by the dotted line illustration, comprising a variable conductance kg in series with a Voltage source e.

Electrons will fiow from the hot cathode to the grid and the grid potential will thus become more negativethan the cathode potential. This electron fiow is blocked when the grid becomes more negativethan the cathode by a critical potential difference which is the contact potential of the grid with the hot cathode." The grid-cathode conductance kg is therefore zero when the grid potential reaches its contact potential value of -e volts. vThe plate-cathode conductance lcp can be reduced to zerofor any given negative potential on the control grid, and the amplifier action of this invention contemplates the adjustment of the plate potential to a value equal to or less than that at which plate circuit conductance is blocked when the grid is more negative than the cathode by' the grid-cathode contact potential e. Examination of the Fig. 1 circuit will show that the grid potential will be reduced if a current fiow is set up, in the proper sense, through the conductance leg. This'condition of current flow can be established by connecting a current source between the gri-dand cathode. Any reduction in the grid potential to a value less negative than that required to block electron fiow, results in av further fiow of electrons to the grid and the ensuing cathode current raises the conductance kg of the grid-cathode space path to a finite value; to carry the input current i, and the circuit balances when the plate-cathode current Ip reaches; the value:

Q2 54k. where gp la is theiexponential current amplification factor and gmis the grid-plate transconductance (mutual conductance) The appea'rance of even an infinitesimal positive grid current results in current fiow in the plate circuit.` i Thischaracteristic arises from the fact that the positive grid current causes the grid to function as a primary ano'de, and the dissipation of the space charge within the grid lattice occasions conduction to the secondary anode or plate. The grid may be at a negative potential with respect to the cathode because its contact potential is greater than the effect of grid current, but it functions as a true anode since it carries a positive current.

A tube circuit such as shown in Fig. l is particularly adapted to the amplification of small currents from sources, such as phototubes, which have zero conductance at zero current fiow. It,l is to be noted that the grid potential varies with the input current that flows through the gridcathode space path, and that the amplifier functions as a true amplifier of this direct current flow.

A practical application of the invention to the amplification of the direct current output o f a phototube is shown in the circuit diagram of I 2. The vacuum tube 4 is shown as a pentode in in which the suppressor grid G8 is internally con.- nected to the cathode K. The screen grid Cfz isV connected to the plate P by a lead 5; and the. tube thus functions as a triode. The control grid G1 has a cap terminal 6 that is connected to the cap electrode 1 of the cathode of phototube 8, the phototube anode being connected to. the tube cathode K through the current source 9. The plate circuit includes the currenty source 2 and measuring instrument 3, as previously described.,

and may also include an adjustable resistance I for introducing plate circuit regulation to control the Shape of the response characteristic.

Typical values for satisfactory operation are indic'ated by the following 'data vrelative to one' amplifier circuit that was constructed and successfully operated. The tube 4 was a type` 3.8` pentode and the phototube 8 Was a type 919. The contact potential of the control grid G1 with the cathode K was about -3 volts, and the plate current source 2 was adjusted to about 6 volts. The phototube current source 9was volts, and the instrument 3 was a microammeter havinga full scale deflection corresponding to 200 microamperes.

The curve H of Fig. 3 was plotted from measurements made with the ppratus just described. The exponential form of theresponse characteristic makes possible the. accurate measurement of small input currents as the output current is still of substantial magnlitude, when the input current is equal to or less'than 1% of the value which produces a full scale defiection of the microammeter. Curve shows. that the output current for 1% relativeillumination wasl about microamp-eres, or 25%, of the maximum output current.

It is to be noted that the amplifier tube does not function in the customary mannriof analternating current amplifier tube. The. minimum plate and screen grid voltage which the. tube manufacturers recommend for the type 38 tube is 100 volts, with the control grid biased to -9 volts with respect to the cathode. IPlate conduction is not blocked under such conditions but, in accordance with this invention, the voltage on the plate and screen grid is at or below that value which corresponds to zero plate conductionwhen the grid is biased negatively by the.grid;-cathode contact potential only. Regulation of the plate voltage below that value Will control the magnitude Of the. responsetO Yenlinput-currfint and;

it iS interesting to, rite. that.: there will besomef ammifiton Whenzth plate voltage. i z.1:.

In actua'l' practice, input.: currntsas; lowas.

aasase 1044 ampei'e will produce a readable defiectio when the microammeter 3 has a range of 0-200 microamperes, and an input of 10-7 ampere will produce a full scale reading. The invention is not restricted to the amplification and/or measurement of any particular range of small direct current values and it will be apparent that currents of an order substantially higher than those noted above may be amplied when tubes having other characteristics and measuring instruments of other ranges are employed.

The invention is distinguished from prior direct current amplifier circuits and methods by the fact that the plate current falls to zero when the grid current is zero, and that a plate current appears with even an infinitesimal positive grid current. The amplifier action has been described with reference to tubes that function as triodes but it Will be apparent that the amplifier tube may operate as a tetrode or other multigrid tube so long as the plate voltage is maintained below that value at which the contact potential of the control grid blocks plate circuit conduction.

I claim:

1. The method of Operating a thermionic tube as a direct current amplifier, the tube having a cathode cooperating with a control grid and a plate, which method comprises maintaining a substantially infinite impedance between the grid and cathode, whereby the grid assumes its normal contact potential with the cathode, impressing between plate and cathode a voltage of not more than that value at which the normal contact potential of the grid with the cathode blocks plate conduction, and passing the current to be amplified through the grid-cathode space path in the direction of grid-to-cathode.

2. The method of Operating a triode as a direct current amplifier which comprises maintaining a substantially infinite impedance between the gridf and cathode, whereby the grid assumes its normal contact potential'with the cathode, impressing between plate and cathode a voltage of not more than that value at which the normal contact potential of the grid` with the cathode blocks plate conduction, and passing the direct current to be amplified through the grid-cathode space path in thedirection of grid-to-cathode.

3. In a direct current amplifier, a thermionic tube having a cathode cooperating with a control grid and a plate, means maintaining the control grid at itsfnormal contact potential with the cathode, a plate-cathode circuit including a current-responsive device in series-with a source of potential, said potential source establishing a plate voltage` of not more than that value atV which the normal contact potential of the grid with the cathode blocks plate conduction, and a:

sourceof direct current to be amplified connectedy between the grid and cathode with the positive,

terminal of the direct current source towardsthe grid, wherehy current flow from said source through the grid-cathode space path dissipates ,thefspace charge at the grid and renders the.Y

plate circuit conductive.

4. In a directv current amplifier, a thermionic tube having a cathcde cooperating with a' control grid and a plate, means InaintainingA the control grid at its normzu contactptentiaiwith 'the cathode, mleans for energizing saidV tube Witha plate-cathode potential of not more than thatvalue at which the normal contact potential. of;

the. grid with the cathode blocks plate conduction, aV current-responsve device connected between plate and cathode, and a source of direct current to be amplified connected between the grid and cathode with the positive terminal of the direct current source towards the grid, whereby current flow from said source through the gridcathode space path dissipates the space charge at the grid and renders the plate circuit conductive.

5. A direct current amplifier circuit comprising a vacuum tube having a control grid cooperating with a cathode and plate, means maintainng the control grid at its normal contact potential with the cathode, a phototube having a cathode and anode, a connection from the phototube cathode to the control grid and a source of direct current connected between the phototube anode and the cathode of the vacuum tube, the positive terminal of said current source being towards the phototube anode, and a plate-cathode circuit including a current measuring instrument and a source of plate potential, said source of plate potential impressing on the plate a voltage of not more than that value at which plate conduction is blocked by' the normal contact potential of the control grid with its cathode.

ROSWELL W. GILBERT. 

